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180                        METALLURGY OF IRON AND STEEL.
ing. If too small an amount of pig-iron is used, the molten bath will contain neither silicon, manganese, nor carbon, and will be viscous and pasty. Such a mass will be oxidized by the flame, and the oxide of iron will scorify the bottom.
SEC. Xb.—Chemical history during melting.—The amount of oxidation during melting is increased by the presence of hydrogen in the gas, by a sharp flame, and by a port construction that allows free air to impinge upon the metal. It is also determined by the manner in which the stock is charged. The pig-iron should be spread evenly over the scrap, so that it will melt first and trickle over the hot steel, and thus each atom of iron will be protected by an atom of silicon or carbon for which oxygen has a 'greater affinity.
It is impossible to obtain perfect protection, and when only a small proportion of pig is used there will be spots where the scrap is entirely uncovered, and large amounts of iron oxide will be produced. If this cinder forms a pool on the viscotis surface of the charge, it will be mixed sooner or later with high-carbon metal, and an interchange will occur with reduction of iron, the result being the same as if mixture had taken place at an earlier stage; but if the fused oxide conies in contact with the hearth, scorification will ensue with formation of silicate of iron, and though at a later period this scoria may be mixed with high-carbon metal, the harm cannot be completely remedied. A portion of the iron may be reduced and a higher silicate formed, but silica once having entered the slag is there to stay, and will permanently hold a greater or less, amount of iron oxide.
The value of the elements found in pig-iron in protecting the scrap from oxidation will be in proportion to their ability to unite with oxygen, as shown by the following table:
1 unit of carbon combines with 1.333 units of oxygen to form CO. 1 unit of silicon combines With'1.143 units of oxygen to form SiOs. 1 unit of manganese combines with 0.291 unit of oxygen to form MnO. 1 unit of titanium combines with 0.176 unit of oxygen to form TiQz,
This table represents a broad truth, but some elements are preferable to others. It is necessary that, after melting, the metal should be protected from the flame by a layer of slag containing about 50 per cent, of silica. If the charge is made up of one^ quarter pig-iron carrying 1 per cent, silicon, the silica produced by oxidation, the sand attached to the pig-iron, and the material